It is known that devices of the plunger type generally comprise a sliding pusher element, or plunger, associated with and elastic member which tends to drive it axially from its guide. The plunger is retained in the inner, or retracted, position by a fusible wire of reduced section which is mounted in parallel with the wire or wires or principal fusible elements of the cartridge in order to melt and release the plunger as soon as these principal fusible wires have melted. The free end of the indicator plunger thus released appears outside the cartridge and indicates to the user that the latter has functioned. In a variation, the release of the plunger may be used to actuate the movable member of a micro-switch which connects in circuit a signalling or effects remote-control.
For such a system to be able to operate correctly, it is, of course, necessary for the fusion of the secondary fuse which retains the plunger against the elastic actuating member, to occur at the level of the base of the plunger which cooperates with this secondary fuse; this is obtained by providing the latter with a portion of reducd section. Under these conditions, the arc and vapours or gases generated by the cut of the retaining fuse may escape to the outside through the annular clearance formed between the outer wall of the plunger and the radial shaft which forms a guide for the slide thereof.
Direct escape of this break-induced arc and of the vapours generated may present risks of damage for apparatus disposed in the vicinity of the fuse cartridge, so that the indicator devices of the above type have had to be provided, at least for certain applications, with a damper arranged so as to oppose the outlet of the arc and to slow down evacuation of the vapours by effecting lamination thereof. Such a damper obviously complicates the construction of which the cost is thus increased, at the same time its presence substantially increases the dimensions of the cartridge, already increased by the alarm micro-switch possibly associated with the indicator device.